When a light body (9) produced integrally with a light main body (3) and a lens (4) is to be produced by using the die slide injection system, the light body (3) having a reflecting surface (14) in the inner surface is produced. The method comprises a reflection surface forming step which vacuum-deposits a reflecting surface (14) on the inner surface of the light main body (3) and which is interposed between a primary injection step for forming the light main body (3) and lens (4), with a fixed mold (2) provided with a vacuum deposition device, and a secondary injection step in which the lightmain body (3) and the lens (4) are disposed end-to-end and integrated together, thus providing a light body (9) having a reflection surface.